There is more to it than just tipping the water bottle tipping hand path. Getting into the scap (elbow drawing back to pinch the scap toward the spine) is an element that my DD was missing, and it lead to ulnar nerve issues that resulted in her unable to throw for 10 weeks as she rehabbed and adjusted her mechanics.
EricF ... what exactly led to the ulnar nerve issues? Was it "getting into the scap", or missing that element, that led to the ulnar issues?
The missing element was not maintaining the tension in the scap long enough during the "bottle tip" part of the throwing sequence. As a result, the way the muscles in the front of her shoulder were being used lead to a condition that irritated the ulnar nerve.
EDIT: This is a great example of doing it right...
We can see the scap tension ("pinch") being maintained as the bottle is being tipped, and the torso is turning forward. My DD was bringing her elbow forward and releasing the scap tension a little bit too soon.